The present invention relates to an electro-hydraulic control for varying the control pressure to a standard controller used to port fluid to the servos of a variable displacement pump.
It is common in designing controls for hydrostatic transmissions to design a distinct control for each desired function. For example, a pressure override (P.O.R.) control is designed to monitor the system pressure of a transmission to protect the transmission from excessive overloads. P.O.R. controls are well known in the art and will be discussed in the present application.
Another common control is an input torque limiter (I.T.L.) or input horsepower limiter (H.P.L.) which matches the torque of a hydrostatic transmission to that of the prime mover. I.T.L. controls generally make use of cams to reset the compensating override pressure for each swash plate position to maintain constant the product of system pressure and pump displacement. Other known I.T.L. controls are hydraulic wherein a pressure drop across a compensating or override spool is maintained proportional to the pump displacement. This is generally accomplished by means of a variable orifice. Other known I.T.L. controls are electrical. In the electrical I.T.L. controls, the displacement of the pump and the system pressure are each measured and then multiplied electrically to produce a signal which is then used to control the displacement of the pump. All of the electrical I.T.L. controls of which applicant is aware make use of a pressure transducer.
In addition, the known controls, as well as those disclosed in co-pending Ser. No. 795,689, require reading pump displacement as an input to the system logic. This requires that at least a portion of the controls be located on the pump, to sense the swashplate position, thus making the system less "flexible" as to the use of remote controls and the relative locations of various system components. Also, sensing displacement electrically results in a DC signal which can be adversely effected by "noise", i.e., stray electrical signals and interference. Finally, sensing displacement is generally relatively more expensive than sensing certain other system characteristics, such as shaft speeds.
Accordingly it an object of the present invention to provide a simple, inexpensive electro-hydraulic controller which can be used to limit or control input torque or horsepower without sensing swashplate position.
In many of the electrical controls of the type discussed above, it is desirable to generate a command signal which is a quotient resulting from the division of one system characteristic by another, wherein either the dividend or the divisor may be a manually selected input, or may be a variable system characteristic. For example, in the electro-hydraulic H.P.L. control disclosed hereinafter, the system logic generates a variable pressure command signal corresponding to maximum system pressure for any given motor output speed. Mathematically, this pressure command signal is obtained by dividing the desired input horsepower (a manual setting) by the motor output speed (a variable system characteristic).
Known electro-hydraulic controls have accomplished necessary division functions in generally one of two ways: either the quotient is approximated, using a linear approximation (or a series of linear approximations), or the quotient is actually calculated by means of an analog divider. The linear approximation method results in relatively poor performance, while the analog division method is excessively expensive for application in hydrostatic transmission controls.
Accordingly, it is another object of the present invention to provide an electro-hydraulic control including a simple, inexpensive method of calculating a "quotient" which is then used by the control as a command signal.